The present invention relates generally to magnetic resonance imaging (MRI) and more specifically, the present invention relates to a single shot magnetic resonance method and apparatus for determining diffusion, flow or motion in a body.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a relatively new approach to medical imaging. MRI is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies that are proportional to a local magnetic field. Radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided for different regions of the volume. These signals are combined to produce a volumetric image of the nuclear spin density of the body being imaged.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have a net spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation, the nuclei gradually return to alignment with the static field and give up energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship (equation 1) that states that the angular frequency, .omega..sub.0, of the precession of the nuclei is the product of the magnetic field, B.sub.0, and the gyromagnetic ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.0 =B.sub.0 .gamma. (1)
Accordingly, by superimposing a linear gradient field, B.sub.z =z.multidot.G.sub.z, on the static uniform field, B.sub.0, which defines the Z-axis, for example, nuclei in a selected X-Y planes can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
A spin echo technique has been employed in obtaining magnetic resonance signals from a body in a nonhomogenous magnetic field. After nuclear spins are tilted and have been processed for a period T, a 180.degree. refocussing RF field is applied to flip the nuclear spins 180.degree.. After a period T, the nuclear spins will refocus, at which time the magnetic resonance signals are detected.
Diffusional processes reflect closely molecular and structural organization. This is one reason behind the rapidly growing interest in use of NMR to measure diffusion in solution, in gases, in solid state, and in vivo. Diffusion procedures are becoming an important class of in vivo procedures, especially as it has been demonstrated that regional differences in diffusion are related to brain damage in acute stroke models. M. E. Moseley, et al., Radiology, 176, 439 (1990). A conventional pulsed field-gradient spin echo (PGSE) diffusion procedure uses a pair of gradient pulses to render a signal sensitive to spin displacement. One conventional method repeats the basic procedure while successively increasing diffusional weighting. The increased diffusional weighting results from repeating the basic procedure n times, with each procedure employing an increased gradient amplitude. Increasing the gradient amplitude creates a spread in phase, causing a decrease in echo amplitude S. S, a function of the gradient amplitude, can be fitted to an equation 2 developed by Stejskal and Tanner: EQU S(G)=S(G=0)e.sup.-.gamma..spsp.2.sup.G.spsp.2.sup.(2.delta.).spsp.2.sup.(.D ELTA.-2.delta./3)D ( 2)
where G is the gradient amplitude, .gamma. is the gyromagnetic ratio, D is the diffusion constant, .DELTA. is an interval between leading edges of diffusion sensitizing gradient pulses, and .delta. is half the duration of each gradient pulse. A difficulty with the conventional system for measurement of diffusion is that resolution or measurement of the diffusion constant D is hampered by bulk motions of the body, and the time-resolution is undesirable because of the procedure's repetition. R. Turner, et al., Radiology, 177, 407 (1990) and C. T. W. Moonen, et al., Science, 250, 53 (1990), both hereby expressly incorporated by reference for all purposes.
The present invention is directed to providing improved time resolution and reduced susceptibility due to bulk motions of the body. The present invention provides a single shot magnetic resonance method and apparatus for measuring diffusion, flow or motion.